It is proposed to study the mechanisms of Zn/Cu, Mn- and Fe-containing superoxide dismutases by mixing solutions of these enzymes with superoxide ion maintained in a water miscible organic solvent such as acetonitrile. A specially constructed stopped flow spectrophotometer will be used in these experiments. The disappearance of super-oxide ion will be followed by the decrease in its absorbance at 250 nm, and the reaction will be studied over a wide range of conditions including temperatures to -30 degrees C and below. These extreme conditions favor the possibility of detecting intermediates in the enzymatic reactions. The methodology of studying enzyme reactions at sub-zero temperatures will be generally explored. Concomitant with the mechanistic studies of superoxide dismutases, the active centers of these enzymes will be probed by a wide variety of physical methods including EPR, ENDOR, ELDOR, NMR, and optical spectroscopies, as well as the techniques of magnetic susceptibility and X-ray crystallography.